FIG. 18 is a perspective view schematically illustrating a prior art optical switch disclosed in IEEE PHOTONICS TECHNOLOGY LETTERS, Vol.2, No.3, pp.214-215, 1990, and FIG. 19 is a plan view thereof. In these figures, reference numeral 180 designates an optical switch. Reference numeral 1 designates an n type InP substrate having opposite front and rear surfaces. An InGaAsP optical waveguide layer 23 including ridge waveguide parts 2a to 2c is disposed on the front surface of the substrate 1. The InGaAsP optical waveguide layer 23 has an energy band gap corresponding to a wavelength of 1.15 .mu.m. This optical switch includes two Y-branch switch parts 3a and 3b and an optical amplifier 17. More specifically, the switch 3a is disposed in a part of the ridge waveguide 2a in the vicinity of the junction between the ridge waveguides 2a and 2b, and the switch 3b is disposed in a part of the ridge waveguide 2c in the vicinity of the junction between the ridge waveguides 2c and 2b. The optical amplifier 17 is disposed in a part of the ridge waveguide 2b. In addition, reference numerals 4 to 6 designate signal lights, and reference numeral 8 designates electrodes of the switches 3a and 3b and the optical amplifier 17.
FIGS. 20, 21, and 22 are sectional views taken along lines 20-20, 21-21, and 22-22 of FIG. 19 illustrating internal structures of the waveguide 2c, the switch 3a, and the amplifier 17, respectively. The internal structures of the waveguides 2a and 2b are the same as that of the waveguide 2c shown in FIG. 20, and the internal structure of the switch 3b is the same as that of the switch 3a show in FIG. 21.
The internal structures of the respective parts will be described in more detail using FIGS. 20 to 22.
In FIG. 20, the InGaAsP optical waveguide layer 12 having a ridge is disposed on the n type InP substrate 1. A p type InP cladding layer 9 is disposed on the ridge of the waveguide layer 12. An InGaAsP cap layer 10 is disposed on the cladding layer 9. The ridge of the waveguide layer 12, the cladding layer 9, and the cap layer 10 are of the same width. The exposed portions of the waveguide layer 12, the cladding layer 9, and the cap layer 10 are completely covered with an insulating film 11 comprising SiO.sub.2.
As shown in FIG. 21, the optical switch 3a further includes an electrode 8 of a prescribed width disposed on a part of the InGaAsP cap layer 10. The surface of the semiconductor structure where the electrode 8 is absent is covered with the insulating film 11.
As shown in FIG. 22, the optical amplifier 17 further includes an InP etching stopper layer 14 and an InGaAsP active layer 13 having an energy band gap corresponding to a wavelength of 1.3 .mu.m, which are interposed between the InGaAsP waveguide layer 12 and the p type InP cladding layer 9.
A description is given of the operation.
First of all, the operation of the switches 3a and 3b will be described. When current is injected from the electrodes 8 of the switches 3a and 3b, the refractive index of the InGaAsP optical waveguide layer 12 in the switch region is reduced due to the plasma effect and the band-filling effect, resulting in a difference in refractive indices between part of the InGaAsP optical waveguide layer 12 included in the switches 3a and 3b to which current is applied and the other part of the InGaAsP optical waveguide layer 12 to which no current is applied. Therefore, the signal light traveling through the waveguide 2a in the direction indicated by the arrow 4 is reflected by the switch 3a and turned to the waveguide 2b due to the above-described difference in the refractive indices. Then, the signal light traveling through the waveguide 2b is reflected by the switch 3b and the reflected light travels through the waveguide 2c (signal light 6). On the other hand, when no current is applied to the switches 3a and 3b, since there is no difference in indices between the switches 3a and 3b and the waveguides 2a and 2c, the signal light 4 is not reflected by the switch 3a but travels straight in the waveguide 2a (signal light 5).
Next, amplification of signal light will be described.
Generally, the intensity of signal light is reduced with an increase in the absorption loss when traveling through the waveguide. In the optical amplifier 7, when current is injected from the electrode 8, signal light passing through this region is amplified by the gain mechanism like that in the operation of a semiconductor laser. Therefore, the intensity of the signal light, which is reduced due to the absorption loss in the waveguide, can be increased again by the optical amplifier 7.
In the above-described optical switch, however, since the switches 3a and 3b and the waveguides 2a to 2c comprise the same semiconductor layers of the same energy band gaps, the variation in the refractive index of the switch at the time of current injection is small, i.e., the ON/OFF ratio of the switch is small, so that the signal light cannot be reflected with high efficiency.
Further, although the optical amplifier disposed in the waveguide compensates for the absorption loss of the signal light traveling through the waveguide, the degree of the amplification is not very high, and the intensity of the signal light cannot be maintained at a high level with high stability.
Meanwhile, Japanese Published Patent Application No. Sho. 60-252329 proposes an optical switch in which a switch part comprises a multiquantum well layer (hereinafter referred to as MQW layer) to increase the ON/OFF ratio of the switch part. In this prior art, however, since the switch part and the waveguide are produced in different process steps, the production process is complicated. In addition, since the switch part and the waveguide include different optical waveguide layers of different semiconductor materials, the transmission loss of signal light at the boundary between the switch part and the waveguide is large, so that the intensity of the signal light cannot be maintained with high stability.
Japanese Published Patent Application No. Hei. 1-283526 proposes an optical switch in which upper and lower cladding layers sandwiching a switch part comprise MQW layers to increase the ON/OFF ratio of the switch part. In this optical switch, differently from the above-described optical switch disclosed in the publication No. Sho. 60-252329, the switch part and the waveguide include the same optical waveguide layer like the optical switch shown in FIGS. 18 and 19, so that the above-described transmission loss of the signal light is avoided. However, since the upper and lower cladding layers sandwiching the switch part are MQW layers, the variation in the refractive index at the switch part is small, and the ON/OFF ratio of the switch part cannot be increased to a desired level.